Method of forming an oxide film

ABSTRACT

A method of forming an oxide film and a method of manufacturing an electronic device utilizing the oxide film is disclosed. A silicon oxide film is formed on a substrate by sputtering. Therefore, the film formation is carried out at a low temperature. The sputtering atmosphere comprises an oxidizing gas and an inert gas such as argon. In order to prevent fixed electric charges from being generated in the film and to obtain an oxide film of good properties, the proportion of argon is adjusted to 20% or less. Alternatively, a gas including halogen elements such as fluorine is added to the above sputtering atmosphere at a proportion less than 20%. Hereupon, alkali ions and dangling bonds of silicon in the oxide film are neutralized by the halogen elements, whereby a fine oxide film is obtained.

This is a division of application Ser. No. 07/966,607, filed Oct. 26,1992, now U.S. Pat. No. 6,586,346, which is a continuation ofapplication Ser. No. 07/650,166 filed Feb. 4, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming an oxide film andfurther a method of manufacturing an electronic device utilizing anoxide film.

2. Description of the Prior Art

In recent years, researchers are attracted by thin film transistorsutilizing non-single crystalline semiconductor thin films.

Conventionally, such a non-single crystalline semiconductor thin film isformed on an insulating substrate by chemical vapor deposition, so thata temperature during the film formation is as low as 450° C. or less.Therefore, soda-lime glass, boro-silicate glass, and the like can beused as the substrate.

The thin film transistor recently attracting researchers is a fieldeffect transistor (simply referred to as FET) having the same functionas that of MOS FET. The size of the thin film transistor is limited onlyby the size of the apparatus to be used for formation of a semiconductorthin film constituting the transistor, so that it is easy to formtransistors on large-sized substrates. Such large-sized thin filmtransistors are promising. For example, the large-sized thin filmtransistors can be used as switching elements of liquid crystal displayshaving a lot of pixels in the form of matrix or switching elements ofone dimensional or two dimensional image sensors or the like.

It is possible to implement a conventional fine processing to thesemiconductor thin films. Hence, the thin film transistor can be formedby means of a conventional fine processing, for example photolithographytechnique. And it is also possible to make the thin film transistorintegrated as a function element of a part of monolithic IC.

Referring to FIG. 2, a typical structure of a conventional thin filmtransistor is schematically illustrated.

Source and drain electrodes 24 and 25 are provided on an insulatingsubstrate 20 made of glass and source and drain regions 22 and 23 areprovided on the source and drain electrodes 24 and 25 respectively and anon-single crystalline semiconductor thin film 21 is provided on thesubstrate 20 and a gate insulating film 26 is provided on thesemiconductor thin film 21 and a gate electrode 27 is provided on thegate insulating film 26.

In the thin film transistor, electric current flowing between the sourceregion 22 and the drain region 23 is controlled by a voltage applied tothe gate electrode 27.

A gate oxide film constituting such a thin film transistor wasconventionally formed by exposing a semiconductor material to thermaloxidation or by thermal CVD under a reduced or atmospheric pressure, orthe like.

Electric characteristics of the thin film transistor largely depend onthe quality of a channel region of the semiconductor film and thequality of the gate insulating film. For this reason, a gate insulatingfilm of particularly good quality has eagerly been required.

In the case of the formation of the gate oxide film by exposing asemiconductor material to thermal oxidation or by thermal CVD under areduced or atmospheric pressure, the temperature during the formation ofthe gate insulating film should be as high as approximately 600° C. inorder to obtain a thin film transistor of good electric characteristics.So that, a heat-resistant substrate material such as quartz glass had tobe utilized though it is expensive.

With respect to a method for forming a gate insulating film at a lowtemperature, a plasma CVD and a sputtering method utilizing an argon gasfor sputtering are well-known. This sputtering method is implemented inan atmosphere comprising a large amount of argon, specifically anatmosphere comprising 100 to 80 volume % Ar atoms and 0 to 10 volume %oxygen. This is because probability of an atom or a cluster of atomsbeing dislodged from a target by collision of one inert gas atom, forexample one Ar atom, is high (in other words, sputtering yield of Ar gasis high). However, in both the plasma CVD and the sputtering methodusing a large amount of argon, the gate insulating film involves numbersof elements (e.g. inert gas elements such as Ar) which was involved in atarget or existed in a chamber during the CVD or the sputtering,resulting in generation of fixed electric charges in the gate insulatingfilm. Further, ions of the elements bombard a surface of an activatedlayer in a thin film transistor and thereby give a damage thereto.Hereupon, a mixed layer of the activated layer and the gate insulatingfilm is formed in the vicinity of an interface between the activatedlayer and the gate insulating film. As a result, interfacial level isformed at the interface and a thin film transistor of finecharacteristics cannot be obtained by any of those methods.

It has been attempted to form a gate insulating film by a photo CVDmethod, and an interfacial level density of the gate insulating film wasabout 2×10¹⁰ eV⁻¹ cm⁻², almost the same as that of a thermal oxidationfilm. However, the photo CVD method required a long period of time, inother words, the film formation speed was extremely slow, so that thephoto CVD method was not suitable for an industrial application.

Referring now to FIG. 7, a network of silicon oxide formed by sputteringin an atmosphere comprising a large amount of argon is illustrated.Symbols 0 in the drawing indicate oxygen or silicon and symbols Xindicate dangling bonds of silicon. A silicon oxide film including agate insulating film is not dense when quantity of fixed electriccharges is large in the silicon oxide film. The larger the number ofdangling bonds of silicon in the silicon oxide film is, the larger thequantity of fixed electric charges is. And the larger the number of Ar⁺in the silicon oxide film is, the larger the quantity of fixed electriccharges is. Ar⁺ and Ar tend to stay inside the silicon oxide network asillustrated in FIG. 7 (Ar⁺ and Ar do not tend to be substituted for Sior 0 in the network). In fact, numbers of dangling bonds of silicon tendto be generated in the silicon oxide film when the silicon oxide film isformed by sputtering in an atmosphere comprising a large amount ofargon. This is partly because internal stress is generated in thesilicon oxide film by Ar or Ar⁺ present inside the silicon oxide networkand partly because defects are formed in the silicon oxide film bybombardment of argon with the silicon oxide film during sputtering.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of forminga dense oxide film by sputtering.

It is another object of the present invention to provide a method offorming a dense gate oxide film by sputtering.

It is another object of the present invention to provide a method ofmanufacturing a thin film transistor of high performance at a lowtemperature.

It is another object of the present invention to provide a method ofmanufacturing a thin film transistor of high reliability at a lowtemperature.

It is another object of the present invention to provide a method ofmanufacturing a thin film transistor of high performance at low cost.

It is a further object of the present invention to provide a method ofmanufacturing a thin film transistor of high reliability at low cost.

An oxide film in accordance with the present invention is formed bysputtering, so that the formation thereof can be carried out at a lowtemperature.

A gate oxide film in accordance with the present invention is formed bysputtering, so that the formation thereof can also be carried out at alow temperature.

The sputtering is implemented in an atmosphere comprising an inert gasand an oxide gas or an atmosphere comprising an inert gas, an oxide gas,and a gas including halogen elements, wherein the proportion of theinert gas is small in the atmosphere. If the inert gas occupies a largeproportion of the atmosphere during sputtering, the formed oxide filminvolves numbers of inert gas elements, which results in generatingfixed electric charges in the oxide film. In particular, in the case ofsputtering in an atmosphere comprising much inert gas of large mass suchas argon, the inert gas bombards the oxide film during the filmformation and causes a lot of defects in the oxide film. As a result,fixed electric charges are generated due to the defects.

When a soda-lime glass, which is cheap, is used as a substrate, a deviceformed on such a substrate should be manufactured at a low temperatureso that the high performance and the reliability of the device are notdegraded by the soda-lime glass. In manufacture of a device comprisingan oxide film, the oxide film may be formed by sputtering in accordancewith the present invention or subsequently may be further annealed bymeans of laser or laser pulse. Further in manufacture of a devicecomprising a semiconductor layer, the semiconductor layer may beannealed by means of laser or laser pulse. The oxide film and thesemiconductor layer are not elevated to a high temperature during thelaser annealing because a laser energy is very concentrated and also thetemperature of the substrate does not exceed 300° C. during the laserannealing, so that a cheap soda-lime glass can be used as the substrate.

Concerning the gate oxide film formed by sputtering, a relation betweenthe proportion of the argon gas during sputtering and an interfaciallevel at the interface between the activated layer and the gate oxidefilm and a relation between the proportion of the argon gas duringsputtering and a flat band voltage were studied. From the study, it wasfound that both the interfacial level and the flat band voltage largelydepended upon the proportion of the argon gas. The interfacial levelexerts an influence upon the performance of the gate oxide film.

FIG. 3 is a graphical diagram showing the interfacial level versus theproportion of the argon gas. The proportion of the argon gas in thiscase means a volume proportion (the argon gas)/(an entire gas comprisingthe argon gas and oxygen (oxidizing gas)) in an atmosphere during theformation of the gate insulating film constituting an insulated gatefield effect transistor by means of sputtering. When the volumeproportion is 50% or less, the interfacial level density of the formedfilm is about 1/10 of that in the case of the use of 100% argonatmosphere as apparent in FIG. 3. FIG. 4 is a graphical diagram showingthe flat band voltage versus the proportion of the argon gas. A siliconoxide film was formed on a silicon semiconductor by the method of thepresent invention, and then an aluminum electrode of 1 mmφ was formed onthe silicon oxide film by means of electron beam deposition, whereby aninsulated gate field effect transistor was completed. The proportion ofthe argon gas in FIG. 4 means a volume proportion (the argon gas)/(theentire gas comprising argon and oxygen (oxidizing gas)) in an atmosphereduring the formation of the silicon oxide film (i.e. gate insulatingfilm) by means of sputtering. The flat band voltage depends on theamount of fixed electric charges existing in the gate insulating film.The flat band voltage tends to be large as quantity of the fixedelectric charges is large. Also, the flat band voltage tends to be smallas quantity of the fixed electric charges is small. As seen in FIG. 4,the flat band voltage corresponding to 0% argon gas atmosphere (i.e.100% oxygen atmosphere) is 1.0V, which is the value of the flat bandvoltage of ideal C-V characteristic (referred to as ideal voltagehereinafter). That is, when the silicon oxide film formation isimplemented in an atmosphere comprising 0% argon (i.e. 100% oxygen), adevice with ideal C-V characteristic can be manufactured.

As described hereinbefore, it is desirable to form a gate insulatingfilm by means of sputtering in an atmosphere comprising less amount ofargon.

When the volume proportion is no more than 20%, flat band voltage isclose to the ideal voltage as shown in FIG. 4. As seen from FIGS. 3 and4, it is preferred that, in the case of the sputtering atmospherecomprising an oxidizing gas and an inert gas, the oxidizing gas shouldoccupy no less than 50%, preferably no less than 80%, typically 100%, ofthe sputtering atmosphere. Also it is preferred that, in the case of thesputtering atmosphere comprising an oxidizing gas, an inert gas, and agas including halogen elements, the gas including halogen elements andthe oxidizing gas should occupy no less than 50%, preferably no lessthan 80%, typically 100%, of the sputtering atmosphere.

Sample A and sample B each of which comprises a P-type singlecrystalline silicon substrate of 1 to 2 Ω·cm, a silicon oxide filminvolving halogen elements formed thereon by the method of the presentinvention, and an aluminum electrode (gate electrode) of 1 mmφ formed onthe silicon oxide film were prepared. The sample A and the sample B werethen annealed at 300° C. With respect to the sample A, BT(bias-temperature) treatment (A) in which a negative bias voltage wasapplied to the gate electrode of the sample A at 2×10⁶V/cm at 150° C.for 30 minutes was carried out. With respect to the sample B, BT(bias-temperature) treatment (B) which was same as the BT treatment (A)except for application of a positive bias voltage in stead of thenegative bias voltage was carried out. The difference between the flatband voltage V_(A) of the sample A after the BT treatment (A) and theflat band voltage V_(B) of the sample B after the BT treatment (B) wasas large as 9V (The difference is referred to as ΔV_(FB)(=|V_(A)−V_(B)|) hereinafter). The reason why the ΔV_(FB) was as largeas 9V is that positive ions such as alkali ions, for example sodiumions, were involved in the samples during the formation of the samples.However, when even a few halogen elements, for example fluorine, wasadded during the formation of the samples, the value of ΔV_(FB) waslargely reduced. This is because the positive ions such as alkali ionswere electrically neutralized by the added halogen elements as shown bythe following formulae.

Besides, dangling bonds of silicon can be neutralized by the addedhalogen elements such as fluorine. It is known that dangling bonds ofsilicon can also be neutralized by hydrogen. However, Si—H bondsobtained by the neutralization are again decomposed by a strong electricfield (e.g. BT treatment), so that dangling bonds of silicon appearagain, resulting in an interfacial level. Therefore, neutralization bythe use of fluorine is preferred.

FIG. 5(A) is a graphical diagram showing a relation between ΔV_(FB) anda proportion of a fluoride gas. Measurement of the ΔV_(FB) was carriedout with respect to samples which had been prepared in the same way asthe samples A and B except that formation of silicon oxide film had beencarried out by sputtering in an atmosphere comprising a fluoride gas andan oxidizing gas. FIG. 5(B) shows a relation between the proportion of afluoride gas and dielectric strength which is defined as a voltagegradient in units of V/cm corresponding to a leak current of 1 μA.Measurement of the dielectric strength was carried out with respect tosamples which had been prepared in the same way as the samples A and Bexcept that formation of silicon oxide film had been carried out bysputtering in an atmosphere comprising a fluoride gas and an oxidizinggas. In FIGS. 5(A) and (B), the proportion of a fluoride gas means avolume proportion (the fluoride gas)/(the entire gas comprising thefluoride gas and the oxidizing gas) in the atmosphere.

There was dispersion in the relation between the proportion of afluoride gas and the dielectric strength. In the graphical diagram ofFIG. 5(B), dielectric strength values and their dispersion ranges (avalues) are shown. When the proportion of a fluoride gas exceeds 20volume %, the values of the dielectric strength of the obtained siliconoxide film are lowered and the a values are increased. So that, theproportion of the added halogen elements is preferably no more than 20volume %, more preferably in the range of 0.2 to 10 volume % in thepresent invention. According to SIMS (Secondary Ion Mass Spectroscopy),the amount of fluorine in the film was measured to be 1 to 2×10²⁰ cm⁻³in the case of adding fluorine at a proportion (fluorine)/(oxygen) of 1volume % during the film formation. From this measurement, it isrecognized that fluorine is an element easily involved in a siliconoxide film when added during film formation by sputtering. However, asrecited hereinbefore, when the fluorine was added too much, e.g. morethan 20 volume %, the obtained silicon oxide film was degraded and thedielectric strength of the film was low with large dispersion.

In the present invention, any of RF sputtering method, DC sputteringmethod, and the like may be adopted as a sputtering method. However, RFmagnetron sputtering method is suitable for the purpose of maintaining astable discharge when a sputtering target is made from oxide of lowconductivity such as SiO₂ or artificial quartz.

As the oxidizing gas used in the present invention, oxygen, ozone,dinitrogen monoxide (nitrous oxide), and the like are preferable. In thecase of ozone or oxygen, oxygen atoms in the ozone or oxygen might beinvolved in an obtained oxide film, however, the oxygen atoms do notcause fixed electric charges in the obtained film since they are mainingredients of the oxide film. Accordingly an extremely fine oxide filminvolving less impurity atoms can be obtained. Besides, since the massof the oxygen atom is less than that of an Ar atom, even if such oxygenatoms collide with an oxide film formed on a substrate during the filmformation, there are few defects caused in the oxide film. Therefore, anexcellent oxide film can be obtained.

With respect to the gas including halogen elements, a fluoride gasselected from the group consisting of nitrogen fluoride (NF₃, N₂F₄),hydrogen fluoride (HF), fluorine (F₂), or fleon gas can be used. NF₃ ispreferable since it is easily decomposed and it is convenient for use.Alternatively, a chloride gas selected from the group consisting ofcarbon tetrachloride (CCl₄), chlorine (Cl₂), hydrogen chloride (HCl), orthe like can be used. The proportion of the gas including halogenelements, e.g. nitrogen fluoride, to an oxidizing gas is preferably 0.2to 20 volume % in the present invention. These halogen elementseffectively neutralize alkali ions such as sodium existing in a siliconoxide film and neutralize dangling bonds of silicon by a heat treatment.On the contrary, if the halogen element added to the silicon oxide filmis too much, there is a possibility that the silicon oxide film issomewhat removed in the form of a silicon-containing gas, for exampleSiF₄. For this reason, the proportion (halogen element)/(silicon) in thesilicon oxide film is preferably 0.1 to 5 atom %.

It is preferred to use materials of high purity during sputtering inorder to obtain a gate insulating film containing less impurities. Forexample, artificial quartz of no less than 4 N, such high purity siliconas to be used as a substrate for LSI, and the like are most preferableas a sputtering target.

Besides, it is preferred that a gas of high purity no less than 5 N isused for sputtering in order to prevent impurities from entering a gateinsulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to (E) show manufacturing steps in accordance with thepresent invention.

FIG. 2 is a cross sectional view schematically showing a conventionalthin film transistor.

FIG. 3 is a graphical diagram showing the interfacial level densityversus the Ar gas proportion during formation of a gate insulating film.

FIG. 4 is a graphical diagram showing the flat band voltage versus theAr gas proportion during formation of a gate insulating film.

FIG. 5(A) is a graphical diagram showing the difference between flatband voltages versus the proportion of fluoride gas during formation ofa gate insulating film.

FIG. 5(B) is a graphical diagram showing the dielectric strength versusthe proportion of fluoride gas during formation of a gate insulatingfilm.

FIG. 6(A) is a schematic view showing a magnetron RF sputteringapparatus used during sputtering in accordance with the presentinvention.

FIG. 6(B) is an explanatory view showing arrangement of magnetic fieldinducing means provided in the apparatus illustrated in FIG. 6(A).

FIG. 7 is an explanatory view showing a network of silicon oxide in theprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment No. 1)

Referring to FIG. 6(A), a planar type magnetron RF sputtering apparatussuitable for use in manufacturing oxide films or electronic devices inaccordance with the present invention is illustrated. The apparatuscomprises a vacuum chamber 31, an evacuation system 32 consisting of aturbo molecular pump 32 b and a rotary pump 32 d respectively providedwith valves 32 a and 32 c, a metallic holder 33 fixed in the lower sideof the chamber 31 for supporting a target 34 thereon, formed with aninner conduit 33 a through which a coolant can flow to cool the target34 and provided with a number of permanent magnets 33 b, an energysupply 35 consisting of an RF (e.g. 13.56 MHz) source 35 a provided witha matching box 35 b for supplying RF energy to the holder 33, asubstrate holder 36 located in the upper position of the chamber 31 forsupporting a substrate 1 to be coated, a heater 36 a embedded in thesubstrate holder 36, a shutter 37 intervening the substrate 1 and thetarget 34 and a gas feeding system 38. Numeral 39 designates sealingmeans for ensuring air-tight structure of the vacuum chamber 31. Inadvance of actual deposition on the substrate 1, impurities occurring inthe targets are sputtered and deposited on the shutter 37 interveningthe substrate 1 and the target 34, and then the shutter is removed inorder to enable normal deposition on the substrate 1. The magnets 33 bare oriented to have their N poles at the upper ends and S poles at thelower ends and horizontally arranged in a circle as illustrated in FIG.6(B) in order to confine electrons in a sputtering region between thesubstrate 1 and the target 34.

Referring now to FIGS. 1(A) to (E), manufacturing steps of a thin filmtransistor in accordance with the present invention are illustrated.

In this embodiment, a cheap soda-lime glass was used as a substrate 1.On the substrate 1, an I-type non-single crystalline semiconductor layerwas formed by a known plasma CVD. Followings are conditions of theformation of the above semiconductor layer.

Substrate Temperature 350° C. Pressure during Reaction 0.06 Torr RFPower (13.56 MHz) 100 W Gas to be Used SiH₄ Film Thickness 2000 Å

The semiconductor layer was patterned into semiconductor islands with ametal mask.

The patterning may be carried out by means of a known photolithographytechnique instead of the metal mask. In FIGS. 1(A) to (E), referencenumeral 2 designates one of the semiconductor islands.

Then, as shown in FIG. 1(B), the non-single crystalline semiconductorisland 2 was crystallized by radiating the island 2 with excimer laserlight 3 to be polycrystalline of large crystal size or singlecrystalline of almost the same crystal size as the size of an elementregion to be formed. Followings are conditions of the radiation of theexcimer laser light 3.

Wave length of Laser Light 284 nm (KrF) Amount of Energy for Radiation200 mJ/cm² Shot Number 10 Pulse Width of Laser Light 30 ns

Then, an N-type non-single crystalline semiconductor layer was formed onthe entire surface of the I-type semiconductor island 2 by a knownplasma CVD method and was subsequently patterned into source and drainregions 4 and 5 as shown in FIG. 1(C). Followings are conditions of theformation of the N-type non-single crystalline semiconductor layer.

Substrate Temperature 250° C. Pressure During Reaction 0.05 Torr RFPower (13.56 MHz) 150 W Gas to be Used SiH₄ + PH₃ + H₂ Film Thickness500 Å

Concerning the above conditions, a large amount of H₂ gas had been usedto dilute the above gas and the RF power had been relatively high, sothat the formed N-type semiconductor layer comprised microcrystals andaccordingly had low electric resistance.

Then, a gate insulating film 6 was formed to be 700 Å in thickness onthe substrate having the I-type and N-type semiconductor layerssuperposed thereon at 300° C. or less in the sputtering apparatusillustrated in FIG. 6(A) by RF sputtering method. After this, the gateinsulating film 6 was patterned by means of photolithography techniqueto thereby obtain contact holes 7 and 8 for contact with the source anddrain regions as shown in FIG. 1(D). Followings are conditions of theformation of the gate insulating film 6.

Target SiO₂ 99.99% Reactive Gas O₂ 100% Pressure during Reaction 0.5 PaRF Power 500 W Substrate Temperature 100° C. Interval between Substrateand Target 150 mm

Properties of the gate insulating film are as follows.

1/10HF Etching Speed 67 nm/min. Dielectric Strength 9.1 MV/cmInterfacial Level Density 2.5 × 10¹⁰eV⁻¹cm⁻²

Then, a gate electrode 9, a source electrode 10, and a drain electrode11 were formed from Al as shown in FIG. 1(E) whereby a thin filmtransistor was completed.

Threshold voltage (simply referred to as V_(th) hereinafter) of such athin film transistor in accordance with this embodiment could be 1V orless. On the contrary, in the case of a similar thin film transistor tothe above except that a gate insulating film thereof was formed under100% Ar gas atmosphere, the threshold voltage could not be 1V or less.

After a gate voltage was applied to the thin film transistor inaccordance with this embodiment in a fixed period, V_(th) thereof wasmeasured. As a result, even after the gate voltage was applied for 1000hours, rate of change of the V_(th) was only about 0.3, that is, therate of change of the V_(th) of the thin film transistor in accordancewith this embodiment was almost the same as that of a thin filmtransistor having a gate insulating film formed by thermal oxidation.From this result, it is understood that localized level in the gateinsulating film 6 and interfacial level between the gate insulating film6 and the semiconductor island 2 were hardly formed.

Mobility of the thin film transistor formed in accordance with thisembodiment was 100 cm²/V·S.

In this embodiment, the gate insulating film 6 was formed by sputteringin an atmosphere comprising 0% Ar gas. However, in the case of forming agate insulating film by sputtering in an atmosphere in which the argonproportion R_(AR) is 0%<R_(AR)≦20%, there were no problems caused onproperties of the thin film transistor. In the case of this R_(AR)range, an interval between the target and the substrate is adjusted tobe a long distance as compared to the case of the 0% Ar atmosphere.Thereby, almost the same quality as that of a gate insulating filmformed by the use of the 0% Ar atmosphere can be obtained.

Further, when the gate insulating film formed by the use of anatmosphere comprising 20% Ar gas or less was radiated with excimer laserlight to thereby be subjected to flash annealing, Ar atoms were removedfrom the gate insulating film and accordingly fixed electric chargeswere decreased in the gate insulating film. In addition, when the amountof energy of excimer laser light directed to the gate insulating filmwas increased, the gate insulating film could be annealed andsimultaneously the underlying semiconductor layers could be crystallizedand therefore the number of manufacturing steps could be reduced, thatis, a step of crystallizing the semiconductor island 2 by means ofradiation of excimer laser light 3 shown in FIG. 1(B) could be omitted.

In this embodiment, a turbo-molecular pump which does not cause aback-diffusion of oils and the like from evacuation system was utilizedin combination with a rotary pump to evacuate a vacuum apparatus forforming a thin film transistor, so that no influences were exerted onthe properties of the gate insulating film and the underlyingsemiconductor layers.

In this embodiment, a thin film transistor of extremely fine propertiescould be formed at a low temperature.

Further, the generation of fixed electric charges in a gate insulatingfilm could be avoided as described hereinbefore, so that it was attainedto provide a thin film transistor of less property change and highreliability for use in a long period of time.

In this embodiment, in order to form a gate insulating film bysputtering, SiO₂ was used as a target. In stead of the SiO₂ target, ahigh purity silicon, e.g. a single crystalline silicon or apolycrystalline silicon, having a purity of 99.999% or more may be usedas a target.

(Embodiment No. 2)

Referring to FIGS. 1(A) to (E), manufacturing steps of a thin filmtransistor in accordance with this embodiment are illustrated.

In this embodiment, a soda-lime glass having a blocking layer such assilicon oxide or silicon nitride provided thereon was used as asubstrate 1. On the substrate 1, an I-type non-single crystallinesemiconductor layer was formed by a known plasma CVD. Followings areconditions of the formation of the above semiconductor layer.

Substrate Temperature 350° C. Pressure during Reaction 0.06 Torr RFPower (13.56 MHz) 100 W Gas to be Used SiH₄ Film Thickness 2000 Å

The semiconductor layer was patterned into semiconductor islands with ametal mask.

The patterning may be carried out by means of a known photolithographytechnique instead of the metal mask. In FIGS. 1(A) to (E), referencenumeral 2 designates one of the semiconductor islands.

Then, as shown in FIG. 1(B), the non-single crystalline semiconductorisland 2 was crystallized by radiating the island 2 with excimer laserlight 3 in a polycrystalline structure of large crystal size or in asingle crystalline structure of almost the same crystal size as the sizeof an element region to be formed. Followings are conditions of theradiation of the excimer laser light 3.

Wave length of Laser Light 284 nm (KrF) Amount of Energy for Radiation200 mJ/cm² Shot Number 10 Pulse Width of Laser Light 30 ns

Then, an N-type non-single crystalline semiconductor layer was formed onthe entire surface of the I-type semiconductor island 2 by a knownplasma CVD and was subsequently patterned into source and drain regions4 and 5 as shown in FIG. 1(C). Followings are conditions of theformation of the N-type non-single crystalline semiconductor layer.

Substrate Temperature 250° C. Pressure During Reaction 0.05 Torr RFPower (13.56 MHz) 150 W Gas to be Used SiH₄ + PH₃ + H₂ Film Thickness500 Å

Concerning the above conditions, a large amount of H₂ gas had been usedto dilute the above gas and the RF power had been relatively high, sothat the formed N-type semiconductor layer comprised microcrystals andaccordingly had low electric resistance.

Then, a gate insulating film 6 involving fluorine was formed to be 1000Å in thickness on the substrate having I-type and N-type semiconductorlayers superposed thereon by the use of a reactive gas includingfluorine at 300° C. or less in the sputtering apparatus illustrated inFIG. 6(A) by RF sputtering method. Subsequently the gate insulating film6 was patterned by means of photolithography technique to producecontact holes 7 and 8 for contact with the source and drain regions asshown in FIG. 1(D). Followings are conditions of the formation of thegate insulating film 6.

Reactive Gas O₂ 95 volume % NF₃  5 volume % Pressure during Reaction0.05 Torr RF Power 500 W Substrate Temperature 100° C. Interval betweenSubstrate and Target 150 mm

Artificial quartz or a high purity silicon, for example a singlecrystalline silicon or a polycrystalline silicon having a purity of99.999% or more was used as a target.

Then, a gate electrode 9, a source electrode 10, and a drain electrode11 were formed from Al as shown in FIG. 1(E) whereby a thin filmtransistor was completed.

Threshold voltage (V_(th)) of such a thin film transistor in accordancewith this embodiment could be 1V or less.

After a gate voltage was applied to the thin film transistor inaccordance with this embodiment in a fixed period, V_(th) thereof wasmeasured. As a result, even after the gate voltage was applied for 1000hours, rate of change of the V_(th) was only about 0.3, that is, therate of change of the V_(th) of the thin film transistor in accordancewith this embodiment was almost the same as that of a thin filmtransistor having a gate insulating film formed by thermal oxidation.From this result, it is understood that localized level in the gateinsulating film 6 and interfacial level between the gate insulating film6 and the semiconductor island 2 were hardly formed.

Mobility of the thin film transistor formed in accordance with thisembodiment was about 100 cm²/V·S.

When a gate insulating film is formed by sputtering in an atmosphere inwhich the argon proportion R_(AR) is 0%<R_(AR)≦20%, an interval betweenthe target and the substrate is adjusted to be a long distance ascompared to the case of forming a gate insulating film by sputtering inan atmosphere comprising 0% Ar. Thereby, almost the same quality as thatof a gate insulating film formed by the use of the 0% Ar atmosphere canbe obtained.

Further, the gate insulating film formed by the use of an atmospherecomprising 20% Ar gas or less may be radiated with excimer laser lightto thereby be subjected to flash annealing. By this flash annealing,halogen elements such as fluorine involved in the gate insulating filmcan be activated and neutralize dangling bonds of silicon, so that fixedelectric charges can be decreased in the gate insulating film.

When the amount of energy of excimer laser light directed to the gateinsulating film was increased, fluorine and sodium involved in the gateinsulating film underwent neutralization by virtue of the energy of theexcimer laser light and simultaneously the underlying semiconductorlayers could be crystallized and therefore the number of manufacturingsteps could be reduced.

In this embodiment, a turbo-molecular pump which does not cause aback-diffusion of oils and the like from evacuation system was utilizedin combination with a rotary pump to evacuate a vacuum apparatus forforming a thin film transistor, so that no influences were given to theproperties of the gate insulating film and the underlying semiconductorlayers.

The halogen element used in this embodiment was fluorine. This isbecause fluorine is active and strongly effects neutralization and hasmass less than other halogen elements. However, chlorine or bromine maybe used instead.

In the present invention, an oxide film can be formed at 300° C. or lessby sputtering. Further in manufacture of the transistor of the presentinvention, all the manufacturing steps can be carried out at 350° C. orless. Due to the formation under such a low temperature, glasssubstrates, e.g. soda-lime glass substrate, can be utilized.

Since other modification and changes (varied to fit particular operatingrequirements and environments) will be apparent to those skilled in theart, the invention is not considered limited to the examples chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention. Examples are as follows.

Although the non-single crystalline semiconductor island was radiatedwith laser to thereby obtain a single crystalline or polycrystallinesemiconductor island in the foregoing embodiments, a non-singlecrystalline semiconductor island which is not subjected to laserradiation may be used in place of the single crystalline orpolycrystalline semiconductor island.

Further, although the transistor formed in the foregoing embodiments wasthat of stagger type as shown in FIG. 1, a transistor of reverse staggertype or an insulated gate field effect transistor constituting amonolithic IC which has a single crystalline silicon layer rather than anon-single crystalline silicon layer can also be manufactured byapplication of the method of the present invention.

Furthermore, a transistor of vertical channel type as well as atransistor of horizontal channel type, e.g. an insulated gate fieldeffect transistor of these types, can also be manufactured byapplication of the method of the present invention.

Although source and drain regions made of N-type semiconductor wereformed in the foregoing embodiments, source and drain regions made ofP-type semiconductor may be formed instead.

Further, the soda-lime glass substrate used in the foregoing embodimentsmay be replaced by other glass substrates, e.g. boro-silicate glasssubstrates, plastic substrates, semiconductor substrates, and conductorsubstrates.

1. A semiconductor device comprising: a substrate comprising plastic; asemiconductor layer over the substrate comprising plastic; a gateinsulating film comprising silicon oxide over the semiconductor layer; agate electrode over the gate insulating film; and source and drainelectrodes over the semiconductor layer, wherein the gate insulatingfilm comprises fluorine and the proportion of fluorine/silicon in thegate insulating film is 5 atom % or less, and wherein the gate electrodeand the source and drain electrodes comprise aluminum.
 2. Asemiconductor device according to claim 1, wherein the semiconductorlayer is crystalline semiconductor layer.
 3. A semiconductor deviceaccording to claim 1, wherein the semiconductor layer is polycrystallinesemiconductor layer.
 4. A semiconductor device comprising: a substratecomprising plastic; a semiconductor layer over the substrate comprisingplastic, wherein the semiconductor layer comprises a N-type sourceregion and a N-type drain region; a gate insulating film comprisingsilicon oxide over the semiconductor layer; a gate electrode over thegate insulating film; and source and drain electrodes over thesemiconductor layer, wherein the gate insulating film comprises fluorineand the proportion of fluorine/silicon in the gate insulating film is 5atom % or less, and wherein the gate electrode and the source and drainelectrodes comprise aluminum.
 5. A semiconductor device according toclaim 4, wherein the semiconductor layer is crystalline semiconductorlayer.
 6. A semiconductor device according to claim 4, wherein thesemiconductor layer is polycrystalline semiconductor layer.
 7. Asemiconductor device comprising: a substrate comprising plastic; asemiconductor layer over the substrate comprising plastic, wherein thesemiconductor layer comprises a P-type source region and a P-type drainregion; a gate insulating film comprising silicon oxide over thesemiconductor layer; a gate electrode over the gate insulating film; andsource and drain electrodes over the semiconductor layer, wherein thegate insulating film comprises fluorine and the proportion offluorine/silicon in the gate insulating film is 5 atom % or less, andwherein the gate electrode and the source and drain electrodes comprisealuminum.
 8. A semiconductor device according to claim 7, wherein thesemiconductor layer is crystalline semiconductor layer.
 9. Asemiconductor device according to claim 7, wherein the semiconductorlayer is polycrystalline semiconductor layer.